Pressure operated rheostat



June 15, 1948. F. G. KELLY PRESSURE OPERATED RHEOSTAT Filed NOV. 2, 1944Gttomcg Snvcntor Kelly Pfederfe G MME] fm Patented June 1s, 194sPRESSURE OPERATED BHEOSTAT Frederick G. Kelly, West Orange,

N. J., asslgnor to Thomas A. Edison, Incorporated, West N. J., acorporation of New Jersey application November 2. 1944, sei-m No.531,62: 11 claim.. (ci. zul-.isi

'I'his invention relates especially to instruments which are responsiveto fluid pressure, and more particularly to pressure-actuated variableresistance devices. s

'I'he present invention is intended for use in remotepressure-indicating systems, and has been especially adapted for use onaircraft. Y

It is an object to provide an instrument of the character mentionedwhich is suiilciently rugsed to permit it to be mounted directly on anaircraft engine.

More particularly, it is an object to provide a highly dependablepressure-responsive resistance device which is capable of withstandingheavy vibration and extremes in temperature without substantialimpairment in life or operation.

It is a further object to provide novel means for dampingpressure-actuated instruments from mechanical vibration and fromfluctuations in applied pressure, and it is a further object to provideeffective and compact means for so damping a pressure-responsivebellows.

It is another object to provide a rugged vibration-proofmotion-amplifying mechanism for coupling a pressureresponsive means tothe movable contact of a rheostat.

It is another object to provide an improved condition-responsiveinstrument which can be adjusted finely for calibration purposes, and itis a further object to provide an adjustable motion-amplifying mechanismof improved character for such an instrument.

A further object is to provide an improved pressure-actuated variableresistance device for the purposes set forth.

Other objects of my invention lie in the combination of parts hereinshown, and will be apparent from the following description and theappended claims.

In the description of my invention reference is had to the accompanyingdrawings of which:

Figure 1 is a view of a resistance device according to my invention,taken principally on a section line through the longitudinal axis of thedevice and with certain parts broken away to better illustrate certainparts of the device;

Figure 2 is an axial view of the end portion of the device of Figure l:

Figure 3 is a sectional view taken on the line 3-3 of Figure 1;

Figure 4 is a fractional View of a portion of the device as it appearsin Figure ibut shown to enlarged scale; s

Figure 5 is a fractional top side view of the mechanism of Figure 4;

Figure 6 is an enlarged sectional view taken on the line B-I of Figure1;

Figure 7 is a fractional right-hand elevational view of the mechanism ofFigure 6;

Figure 8 is a sectionalview taken on the line l-t oi' Figure 7;

Figure 9 is a sectional view taken on the line O-l of Figure 8.

The principal components of the pressureresponsive resistance deviceshown in Figure 1 comprise a rheostat Il; a pressure-responsiveactuating means for the rheostat shown, in its preferred form. as abellows il; a plunger I2 actuated by the bellows; anda motion-amplifyingmechanism coupling the plunger tothe rheostat and generally referred toas Il.

The housing for this resistance device is a rigid cylindrical structurecomprising a cupshaped housing member Il in which is disposed therheostat and motion-amplifying mechanism; a screw plug Il which isthreaded interiorly into the open end portion of the housing member M;and a casing I8 of suitably lesser diameter than the housing member I4which houses the bellows Il and which is threaded into a bore l1 of theplug I5, the casing being locked to the plug Il by a lock nut l2 whichis tightened against 'the base oi' the plug. Onthe outer end of thecasing there is an exteriorly-threaded nipple I9 for pipe connection ofthe casing to a source ci' pressure to be measured.

The bellows Il is' sealed fluid-tight at 20 to the inner end oi' asleeve 2| which fits the 'bore Ila of the'nlpple I s, the sleeve havinga peripheral shoulder 2Ia which seats in an inner end portion of thisbore to hold the sleeve against outward movement from the casing.Engaging a closed inner end` of the bellows is a cap 22 provided on anend of the plunger l2. The plunger is slidably mounted in a bearing 2lbwhich is carried in "a sleevev member 23 that engages a reduced-diameterbore 24 at the end of the plug l5. This sleeve member has a peripheralflange 23a which -is held firmly against the inner end of the bore I1 byriveting' of the sleeve member to the plug as at 22e.l Interposedbetween'the cap 22 and the flange 23a is a compression spring 2l whichurges the bellows Il into a contracted condition. When fluid underpressure is introduced through the nipple I9 into and out of thebellows, Athe bellows is expanded and contracted lengthwise according tothe pressure of that fluid and the plunger!! is moved back and forthalong the longitudinal axis of the housing. By this movement oi' theplunger the rheostat I is operated asis hereinafter explained.

Preferably, there is employed a rheostat of the rotary contact type asshown. This rheostat comprises a single-layer winding 26 of Nichromewire having its longitudinal axis curved on an arc through the majorpart of a circle, a central shaft 21 and a spring contact member 28carried by the shaft 21 and having an arm 28a slidably contacting thewinding 26. The rheostat is mounted with its shaft 21 transverse to thelongitudinal axis of the housing and in the plane of the axis of theplunger I2. This is journalled in two standards 29 and' 30, of which.

the standard 29 is integral with a. base 3| that is held by screws 3Iato the end of the plug I5 and the standard 30 is held by screws 38a tothis base. As shown in Figure 6, the base 3| is' circular and is cutaway at 3Ib to clear the plunger I2 and other mechanism as willvhereinafter appear. The winding 25 is carried by a metal block 32 .whichis held by screws 33 against the inner side wall of the standard 30. Theblock 32 is principally circular, having only a substantiallysector-shaped recess 34 for clearing the shaft 21 as shown in Figure 4.The winding 26 is provided with a nat insulating core 35 and is applied,through an intervening insulating strip 36, to the arcuate periphery ofthe block 32, the projecting end portions of the core 35 being securedby screws 31 to the block for holding the winding in place.

The spring contact member 28-.which malr generally be referred to as theindicating means of the instrument-is held in an insulating bushing 38that is staked to a. flanged collar 39. This collar is mounted on theend portion of the shaft 21 which extends beyond the standard 30 asshown in Figure 8. For holding the contact member tightly to the shaftin adjustable angular positions relative thereto, the collar 39 has areduceddiameter sleeve portion 40 which is provided with mentionedcomprises a. pinion 44 secured to the shaft 21 at a point thereonbetween the standards 29 and 30 and a lever 45 having a gear segment asegmental recess 40a as shown in Figure 9,

and this outer end portion 40 is embraced by a clamping ring 4I that isheld by a set screw 42 to the shaft 21, the set screw passing radiallywith respect to the shaft through the recess 40a and engaging a fiat 43on the shaft. When the setscrew is just slightly loosened from theshaft, the clamping ring is released from the sleeve 40 and the contactmember is free to be adjusted relative to the shaft, it being understoodthat the clamping ring will be retained in its angular position relativeto the shaft as the contact member is so adjusted because of theengagement of the set screw with the at 43. The range of angularadjustment of the contact member 28 is determined by the` angular widthof the recess 40A, this width being suitably just slightly less `than180 as shown .in Figure 9. Into whatever position the contact member isso adjusted within this range, it is locked to the shaft by tighteningthe set screw 42 of the clamping ring. The advantage in this particularmeans for adjustably locking the contact member to the shaft 21 is thatthe contact member can be adjusted repeatedly without marring any partsthat will reduce the accuracy of adjustment since the set screw engagesthe same point on the shaft 21 for each adjustment; also, since the setscrew is on a radius line, there is no tendency for the clamping ring orcontact member to be displaced by force exerted against the set screw intightening it against the shaft.

46 meshing with that pinion. The lever 45 lies adjacent the standard 29and is secured to a shaft 41 which has reduced-diameter end s pivoted tothe standards 23 and 30, the pivot axis of the lever being transverse tothe path of movement of the plunger I2 and offset therefrom as shown inFigure 4. The lever 45 has a side arm extending transversely to theplunger I2, which consists of a screw 48. This screw-wherein ishereinafter referred to as an armscrew-is threaded into a block 49 (seeFigures 4 and 5) that is held to the shaft 41 in fixed relation vto thelever, the block being split as shown in Figure 5 so as to clamp thethreads yof the arm screw and hold it tightly in place. On the end ofthe arm screw there is a circular concentric head 50 the peripheral edgeof which confronts a nat smooth-surfaced end face 5I on the head of avscrew 52 that is threaded axially into the plunger.

By means of a torsional spring 65, which angularly biases the shaft 21,the peripheral edge of the head of the arm screw 48 is maintainedcontinuously in contact with the face 5|.

It will be understood that in view of the vlever 45 being pivoted on axed axis at the side of the end face 5| of the plunger I2, the leverageof the arm screw 48 is varied as the screw is threaded inwardly andoutwardly with respect tothe block 49. For example, when the arm screwcontacts the central portion of the face 5| as shown in Figure 4, agiven displacement of the plunger I2 will turn the lever 45a givenamount, and when the arm screw is adjusted to shorter and longer lengthsthe same displacement 'of the plunger will produce respectively greaterand lesser angular displacements of the lever 45. Similarly, from adynamic standpoint, when the arm screw is adjusted to shorter and longerlengths the rate of movement of the lever 45 for a given rate ofmovement of the plunger is respectively increased and decreased. Theaction of the arm screw 48 is thus to control the rate or sensitivity ofresponse of the instrument.

In calibrating the instrument several different adjustments are made.For example, not only is the arm screw 48 adjusted to set the rate ofresponse as aforementioned, but also the contact member 28 is adjustedrelative to the shaft 21, by the means hereinbefore described, to setthe instrument to zero, it being understood that this setting is carriedout by adjusting the arm 28a along the winding 26 to a zeroorpredetermined reference-indicating pressure while the bellows I I issubjected to a zero or reference pressure.

In addition to the foregoing adjustments there is required an adjustmentof predeterminately setting the effective angle between the plunger I2and lever 45 for a given state of actuation of the bellows I I, thisangle being that between the direction of movement of the plunger and aline running through the pivot axis of the lever 45 andl the point ofcontact of the arm screw 48 with the end face 5I of the plunger. Thisangle isv of importance because it varies as the plunger moves and hasthe effect of giving a non-linear respome to the instrument to theextent of the variation of the cosine of the angle. In a limited rangeof movement of the lever 45 from a position of the lever wherein thisangle is this position being herein termed the midrange position of thelever-the movement- The motion-amplifying mechanism I3 afore- 75transmitting ratio between the plunger I2 and lever 46 is substantiallyconstant. It is therefore desired that the working range of the lever llshall extend clockwise and counterclockwise from this mid-range positionby equal amounts so that the contact arm 28a shall be movedsubstantially linearly with the pressure applied to the bellows Il. Withdifferent instruments, this angie will be different for a given pressureapplied to each instrument because of manufacturing variations in thecomponents of the instruments. To compensate for these manufacturingvariations, this angle is predeterminately set for a given pressureapplied to each instrument by adjusting the screw 52 relative to theplunger i2 and by correspondingly adjusting the contact arm 28a relativeto the shaft 2l to set the arm to a predetermined position with respectto the winding 26.

The operation of the mechanism herein so far described is as follows: Asthe pressure is increased within the bellows ii the plunger i2 is movedalong the axis of the instrument and, through contact of the end face 5iwith the head II of the arm screw 48, the lever 4l is-turned in acounterclockwise direction as seen in Figure 4, the maximum range ofmotion of the lever being of the order of 35. This motion of the leveris amplified many times by the gear I8 and pinion 4I, and transmitted byshaft 21 to the contact member to cause the arm 28a to slide along thewinding 28 through a maximum angular range of the order of 180. As aresult a varying resistance is produced between the ends of the winding28 and the contact arm 28a in accordance with the rise of pressurewithin the bellows. As the pressure in the bellows is reduced, thereverse movements take place: the plunger being returned by the spring25 and the contact arm 28a being correspondingly returned along thewinding 28 by the spring 85. By appropriate electrical connection of therheostat i to a suitable remote indicating instrument. not shown, theresistance variations of the rheostat are translated into pressureindications at a remote point.

For making electrical connection from a remote point to the rheostat I0,lead wires 54 are connected to lugs 55 at the ends of the winding 2t andto the contact arm 28a-the connection to this arm being in the mannerhereinafter explained-and are brought out through an insulating grommetIl in the end wall of the housing member Il. The lead wires connectrespectively to terminals l1 carried by an insulating plate I8, thisplate being spaced from the end of the housing member i4 by insulation58. At the outer side of this plate there is a cable 80 having leadwires 6i connecting respectively to the terminals l. Secured to the endportion of the cable is an annular body 62 of rubber. This body issurrounded by a metal shell B3 which its the housing member`|4 and isheld thereto by `screws 84.

In general, instruments which are to be used on aircraft have to be ableto withstand heavy vibration and extremes of temperature. v The presentinstrument is adapted to withstand not merely the vibration andtemperature extremes ordinarily required in aircraft use, but a yet morerigid and severe degree of vibration and temperature variation so thatthe instrument may be mounted directly on the engine. A typicalapplication for such instrument is for measuring engine oil pressure.Important advantages in directly mounting a pressure-responsiveresistance device of a remote pressure-indicating system on the engineis that no oil line (capillary tubing) is requiredthe usual long lengthoi' oil line between the engine and the remote indicating instrumentbeing here replaced by wiring-the rate of response of the indicatinginstrument to changes in pressure at the engine is greatly increasedbecause the delay inherent in a long length of capillary tubing is hereavoided. the weight of the measuring system is materially reduced, anddependable operation is assured during ying in arctic climates or in thestratosphere because there is no oil line to freeze or congeal. f

In proofing the instrument against temperature, itis found to beimportant that the spring 2i have an essentially zero temperaturecoefilcient of expansion. A satisfactory material for this spring is onemade of that known commercially as Elinvan Since the bellows ii is arelatively flexible component in the system, it need not be made of anyspecial material having a low temperature coeilicient of expansion. Inthe coupling mechanism Il it is important that none of the elementsshall freeze in position at one temperature extreme nor that they shallhave inordinate play at the other temperature extreme. Important in thisrespect is the elimination of as many positive connections between theelements as possible. To this end the biased contact of the arm screw 4Iwith the head of the plunger i2 is helpful.

To meet the rigid requirements with respect to vibration, it has beennecessary to employ a number of expedients. First, it may be notedbutwithout respect to relative importance-that in order to anchorpositively the cable il to the housing, it is highly important that therubber head l2 be made integral with the sheath 62a of the cable. In theinstrument proper, it is important that the plunger i2 have not lessthan a predetermined minimum area of sliding contact with the sleeve 23and that the sleeve shall have a hard and entirely friction-free bearing23h for supporting the plunger. To this end the plunger is provided witha diameter at least of the order of .175 and the bearing 2lb is made ofOilite bronze, or equivalent material. Also, in order to maintain acontinuous contact between the contact arm 20a and the resistancewinding 26, it is found necessary that the arm have a small effectivemass and a high degree of stiffness so that it shall not break contactwith the winding at less than a peak vibration acceleration of timesgravity.

Moreover, it is essential thatall rotary eiements of the couplingmechanism shall be suitably balanced. To this end, the contact member 28is provided with an integral extending portion 28h (see Figure 7) whichis opposite the contact arm 28a and the spring arm 68 hereinafterdescribed, and the lever 45 is provided with a counterweight 48a asshown in Figure 8.

In addition to the foregoing, it is essential that the shafts Il and 2lbe biased longitudinally so that they will not vibrate longitudinally tocause excessive wearbetween the gear segment 4I and the pinion u or tocause the contact arm 2l to breakfcontact with the winding 26. Thelongitudinal biasing of shaft ll is effected by a compression spring 52which surrounds the shaft and is interposed between lever 45 andstandard 20 as shown in Figure 8. The longitudinal biasing of the shaft2l is provided by action of the spring arm which is integral i of theaforementioned sleeve 2l.

with the contact member 28, -against a stop arm 61 held by screws 68through spacers 69 to the standard 30. The spring arm 66 is bent in asubstantially half circle and has a contact button at its end portionwhich bears against the stop arm 61 at a point axially in line with theshaft 21, the advantage in obtaining the biasing of the shaft from apoint axially in line therewith being to eliminate friction.

A further important feature in rendering the present instrumentinsensitive to mechanical vibration lies in the provision of novel andeffective means for damping the bellows Il This damping means -isV of aclosed` liquid-filled type, and is novelly associated with the bellowsby being disposed therewithin, the advantage in this association beingto reduce the over-all size of the instrument. The damping meanscomprises an expansibie bulb 13 of a material such as synthetic rubber,which is mounted onto a screw plug 14 that threads into the outer endportion When this bulb is in an unexpanded state it has an oblong shapein cross section as shown in Figure 3. The open end or neck portion ofthe bulb is circular in transverse section and fitted into an annulargroove 15 provided by the end wall of the plug 14, and the outer wall ofthis groove is spun radially inwardly, at a short distance 'back fromthe end of the plug, to form an annular constriction 15a for tightlyembracing the neck of the bulb and securely sealing the plug to thebulb. In the plug there is an axial passageway 16 for transmitting fluidpressure to be measured-typicaily the oil pressure of an aircraftengineinto the bulb.

The bulb 13 and bellows ll enclose a liquidtight chamber 11. Thischamber is divided into two compartments 11a and 11b by a partitionwhich is in the form of a rigidly-walled closedended tube 18, this tubebeing interposed between and spaced from the bulb and bellows and beingsealed at its base to the inner portion of the sleeve 2l as shown inFigure l. 'I'he tube 18 has a restricted opening 19 which may forexample be a saw cut at a corner portion of the tube of say .005" widthand .010 length. The chamber 11 is completely filled with apressuretransmitting and damping fluid that will maintain a liquid stateand have negligible'variationy volumetric variation of the bulb 13,between its flattened shape shown in Figure 3 and a nearly cylindricalshape, is however greaterthan the sum of the thermal volumetric changeof the damping .iluid and the displacement volume of the bellows Ilbetween contracted and expanded conditions. Thus, by filling thecompartments 11a and 11b with a proper amount of damping fluid, the bulbwill have a flattened partially collapsed shape shown in Figure 3 at thehighest operating temperature and with the bellows II contracted, andwill take a substantially cylindrical shape but be not stretched at thelowest operatingstemperature and with the bellows Il expanded. 'Iheonlypressure consumed by the bulb in changing between its flattened andcylindrical shapes is that required to flex the walls of the bulb. whichis a negligible pressure of only approximately .25 lb. per sq. in.Accordingly, the bulb has no substantial effect on the sensitivity orcalibration of the instrument.

The damping action of the system just above described depends upon theincompressibility of the liquid which fills the chamber 11 andA therestricted rate with which the liquid can flow through the opening 19between compartments 11a and 11b of this chamber. For example, when thepressure-responsive device is subjected to vibration the bellows l I,having resilience and mass, will in the absence of damping respond tothat vibration and cause the contact arm 28a to vibrate. the response ofthe bellows being particularly great when the frequency of the forcedvibrations approaches a natural or resonant frequency of the bellowssystem. With the present damping system, however, the bellows canrespond only as much as it is permitted to by flow of liquid into andout of compartment 11b through the restricted opening 19. The frequencyof vibration to which the instrument is subjected is, for aircraftapplications, typically atleast as high as 45 C. P. S. with manyharmonics of the vibration being at frequencies much higher than this.At frequencies of 45 C. P. S. or more, the resistance of `the restrictedopening ishigh and the damping of the bellows is accordingly veryeffective. 'I'his damping is likewise effective as to rapid fluctuationsin applied fluid pressure to the instrument for as the fluid pressuretluctuates the restricted ilow rate of the opening 19 will eiectivelyprevent transmission of those fluctuations to the bellows Il. The normalvariations in fluid pressure which are to be measured are, however,relatively slow, being typically of the order of 1 C. P. S. or less. Therestricted opening 19 passes liquid at a sufficiently fast rate toenable these normal pressure variations to betransmitted to the bellowsi I without appreciable lag. Accordingly, the bulb 13 will undergovolumetric variations in response to the normal pressure variations, aswill also the bellows I I, with the result that the rheostat l0 isvaried in resistance in accordance with the normal pressure variations.

The electrical connection of the lead wire to the contact arm 28ahereinbefore mentioned ls made conveniently through the stop arm 61 andspring arm 66. Accordingly, the arm 61 is insulated from the screws 6Band spacers 69 by insulating bushings and washers generally referred toas 1I, and one of the lead wires 54 is soldered to the member 61 as ata. point 12 shown in Figure 7.

The embodiment herein particularly shown and described is intended to beillustrative and not limitative of my invention. since the same issubject to many changes and modifications without departure ,from thescope of my invention, which I endeavor to express according to thefollowing claims.

I claim:

1. A closed pressure-transmitting system for operation withinpredetermined pressure and temperature ranges comprising apressure-actuatable member, a flexible bulb connected -to said member toform a closed space, said space containing a pressure-transmitting iluidcharacterized as having a thermal coeillcient of expansion, means forapplying pressure to said bulb to actuate said member, said bulb beingin its state of maximum collapse when said applied pressure is at oneextreme and the temperature of said fluid is at the opposite extreme oi'their respective ranges. and the volumetric displacement of said bulbbetween said condition of maximum collapse and a totally uncollapsedcondition being equal at least to the sum of the thermal volumetricchange of said fluid through said temperature range and of thevolumetric displacement of said actuatable member through the range ofsaid applied pressure.

2. In a pressure-actuated variable resistance device includingpressure-responsive means: the combination of an element actuated bysaid pressure-responsive means along a predetermined path; a rheostathaving a movable contact member; a pivoted lever coupled to said contactmember; an arm associated with said lever and contacting said actuatedelement at a point oii'set from said pivot axis in directions transverseto said path; and means for adjusting the effective length of said armwith respect to said lever whereby the rate of turning of said contactmember is controlled for a given rate of movement of said actuatedelement.

3. A pressure-actuated variable resistance device comprising arectilinearly-movable pressureresponsive member; a rheostat having ashaft and a contact member pivotally carried thereby; a pivoted levercoupled to said shaft; an arm associated with said lever; and a head onsaid pressure-responsive member for contacting said arm and turning thesame in response to rectilinear movement of the member, said contactmember being angularly adjustable on said shaft and said head beingrectilinearly adjustable with respect to said pressure-responsive memberwhereby the angle between said arm and the direction of movement of saidpressure-responsive member can be set to a predetermined value for anygiven position of said contact member.

4. In an instrument including operable means and actuating meanstherefor: a motion-transmitting mechanism intercoupling said actuatingand operable means and comprising independent members coupled to saidactuating and operable means respectively, said members being biasedVinto contact with `one another; means pivotally holding one of saidmembers on a predetermined axis and means holding the other of saidmembers for movement in a path transverse to said axis and oiIsettherefrom, one of said members being mounted for adjustment along saidpath.

relative to its mounting means whereby the angle between said path andthe line passing through said axis and the point of contact between saidmembers is predeterminately established for a given position of saidactuating means.

5. In a pressure-actuated variable resistance device includingpressure-responsive means: the combination of an element actuated bysaid pressure-responsive means and having a smooth driving face arheostat having a shaft and a contact member carried pivotally thereby;and means for coupling said contact member to said actuated elementincluding a lever drivingly connected to said contact member and havingits pivot axis offset laterally from the path of said actuated element,a screw-threaded member associated with said lever and having aconcentric circular head the peripheral edge of which is adapted tocontact said face, said screw-threaded member being adjustable relativeto said lever along said face; and means biasing said leverto maintainsaid screw-threaded member in contact with said face.

6. In an instrument including operable means 10 and actuating meanstherefor: a motion-amplifying mechanism for coupling said operable meansto said actuating means, said mechanism including a member pivoted on apredetermined axis and operating said operable means as the member isturned, n member coupled to said actuating means and actuated thereby,and an adjustable coupling between said members comprising asmooth-surfaced face on one of said members and moved thereby intransverse relation to said axis and in a path spaced therefrom, and alscrewthreaded element secured to the other of said members and havingits longitudinal axis in transverse relation to said path, said elementhaving a concentric circular head whose peripheral edge is held incontact with said face, and said element being threadingly supportedwhereby with turning of the element the distance is varied between saidpivot axis and the point of contactv of the peripheral edge of theelement with said face.

7. In an instrument including operable means and actuating meanstherefor: an adjustable mechanism for coupling said operable means tosaid actuating means, including a member pivoted on a predetermined axisand causing said operable means to be operated as the member is turned,a member coupled to said actuating means and moved thereby, a firstscrew-threaded element secured to one of, said members and hav-ing anend face at right angles to the direction of movement of the element,and a second screw-threaded element secured to the other-of said membersand disposed in transverse relation to the path of said firstscrew-threaded element having a circular concentric head the peripheraledge of which confronts said face, and means biasing one of said memberswhereby the peripheral edge of said head is maintained in contact withsaid face.

8. A pressure-actuated variable resistance device adapted for directmounting on an aircraft engine, comprising a pressure-responsive member;a rheostat having a balanced contact member pivoted on an axis at rightangles to the direction of movement of said member; a pinion connectedto said contact member; a balanced gear member meshing with said pinion;lever means coupling said pressure-responsive member to said gearmember; and means for holding said contact member and gear memberconstrained against axial play.

9. A pressure-actuated variable resistance device adapted for directmounting on an airplane engine and to withstand the normal enginevibration, comprising a pressure-responsive means, a rheostat includinga resistance element and a relatively-movable contact arm associatedtherewith, and balanced rotary means for coupling said contact arm tosaid pressure-responsive means, said contact arm being biased intoengagement with said resistance element and having an effective mass andstiifness adapted to prevent the arm from being flexed out of contactwith said resistance element when said device -is vibrated.

10. In an instrument of the character described: the combination of arheostat including a journalled shaft and a contact member carried bysaid shaft; means for turning said shaft; and means for holding saidshaft constrained against longitudinal play comprising a stationary stopstop members. i

FREDERICK G. KELLY.

REFERENCES CITED The following references are of record in the ie ofthis patent:

UNITED STATES PATENTS Name I Date Gai'sed May 31, 1881 Number NumberNumber 12 Name Date Y Fulton Sept. 1A 1908 Lester May 14, 1912 SwallenAug. 8, 1933 Weatherhead Nov. 23, 1937 Harrison May 17, 1938 Paul Dec.24, 1940 Addy et al July 8, 1941 Liner May 19, 1942 Allen Oct. 6, 1942FOREIGN PATENTS Country Date Germany 1935 Certificate of CorrectionPatent No. 2,443,252. June 15, 1948.

FREDERICK G. KELLY It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctlonas follows:

Column 10, line 35, claim 7, after the word element insert the comma andWords said second screw-threaded element;

and that the said Letters Patentshould be read with this correctiontherein that the same may conform to the record of the case in thePatent Oflce.

Signed and sealed this 9th day of August, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommznmr of Patents.

Certificate of Correction Patent No. 2,443,252. June 15, 1948.

FREDERICK G. KELLY It is hereby certied that erro1 appears n the printedspecification of the above numbered patent requiring correctlon asfollows:

Column 10, line 35, claim 7, after the word element insert the comma andwords Y said second screw-threaded element;

and that the said Letters Patent` should be read with this correctiontherein that the same may conform to the record of the case in thePatent OHce.

Signed and sealed this 9th day of August, A. D. 1949.

THOMAS F. MURPHY,

esistant ommz'naner of Patents.

